Pigment reclaiming

ABSTRACT

A METHOD AND APPARATUS FOR PHOTOELECTROPHORETICALLY SEPARATING PIGMENT PARTICLES DISPERSED IN A SUSPENSION. THE INVENTION MAY BE USED FOR SEPARATING ELECTRICALLY PHOTOSENSITIVE PARTICLES HAVING DIFFERING SPECTRAL RESPONSES OR SEPARATING ELECTRICALLY PHOSENSITIVE AND RELATIVELY NONELECTRICALLY PHOTOSENSITIVE PARTICLES.

Jan. 11 1972 v, TULAGIN EI'AL PIGMENT RECLAIMING Filed Jan. 28, 1969 2 Sheets-Sheet 1 FIG. 1

INVENTURS, v-63 A I BY VSEVOLOD SXA IH JtOV A TTORNE Y Jan. 11, 1972 TULAGIN ETAL 3,634,221 PIGMENT RECLAIMING Filed Jain. 28, 1969 2 Sheets5heet United States Patent US. Cl. 204-180 R 7 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for photoelectrophoretically separating pigment particles dispersed in a suspension. The invention may be used for separating electrically photosensitive particles having diifering spectral responses or separating electrically photosensitive and relatively nonelectrically photosensitive particles.

BACKGROUND OF THE INVENTION This invention relates in general to photoelectrophoretic separation processes. This application is a continuation-inpart of our copending application Ser. No. 589,930, filed Oct. 27, 1966.

Recently, there has been developed an electrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in U.S. Pats. 3,383,993 to Shu-Hsiung Yeh, 3,384,565 to V. Tulagin et al. and 3,3 84,566 to H. E. Clark, all issued May 21, 1968. In such imaging systems, variously colored light absorbing particles are suspended in a non-conductive liquid carrier. The suspension is placed between electrodes, subjected to a potential difference and exposed to an image. When these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of this system is the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation, through interaction with one of the electrodes. In a monochromatic system, particles of a single color are used, producing a single colored image equivalent to conventional black-and-white photography. In a polychromatic system, the images are produced in natural color because mixtures of particles of two or more different colors which are each sensitive to light of a specific wave-length or narrow range of wavelengths are used. Particles used in this system must have both intense pure colors and be highly photosensitive.

In a subtractive polychromatic system, the particles are selected so that those of different colors respond to different wave-lengths in the visible spectrum corresponding to their absorption bands. Also, it is important that the particles do not have substantial overlap in their spectral response curves thus allowing for color separation and subtractive multicolor image formation. In a typical subtractive multicolor system, the particle dispersion should include cyan colored particles sensitive mainly to red light, magenta particles sensitive mainly to green light and yellow particles sensitive mainly to blue light. When mixed together in a carrier liquid, these particles produce a black appearing liquid. When one or more of the particles are caused to migrate from one electrode toward the other electrode, they leave behind particles which produce a color equivalent to the color of the impinging light. Thus, for example, red light exposure causes the cyan colored 3,634,221 Patented Jan. 11, 1972 particles to migrate, leaving behind the magenta and yellow particles which combine to produce red in the final image. In the same manner, blue and green colors are reproduced by removal of yellow and magenta, respectively. When white light impinges upon the mix all particles migrate, leaving behind the color of the white or transparent substrate. No exposure leaves behind all particles which combine to produce a black image. This is an ideal technique of subtractive color imaging in that the particles perform the dual functions of final image colorant and photosensitive medium.

Excellent images may be produced utilizing a wide variety of particles consisting either of a single pigment or a composite of several ingredients. However, it has been found that many pigments or composite particles as purchased commercially and milled to appropriate sizes or as synthesized, contain impurities which degrade images produced therefrom. The primary problem has been found to be undesirable spectral response in some of the particles in an imaging mix. Where some of the particles of a given color respond to a broader range of wave-lengths than would be desired, these particles Will migrate from the base electrode in response to the improper light thereby decreasing density of that color in the final image. On the other hand, where the particles fail to respond to light of the proper wave-length, these particles will remain on the base electrode while most of the particles of that color migrate away. Thus, in the final image, an undesired color cast will remain in inappropriate areas. For example, if a portion of the cyan colored particles are not fully responsive to red light, they will not migrate when struck by red light and will remain on the base electrode. This area then, which should appear red in the final image, will have a bluish cast due to a portion of the cyan particles remaining with the magenta and yellow particles.

It has also been found that some particles of a given color are more negative and others more positive than the average. It is preferred in an imaging mix that all the particles have a single polarity so that they will initially adhere to the base electrode and migrate away therefrom upon exposure to light. If the polarity of some of the particles of a given color is incorrect, they may either adhere too strongly to the base electrode and not migrate when struck by appropriate light or may migrate away from the base electrode when not contacted by appropriate light. These characteristics will either produce an incorrect color balance or lower color contrast.

Thus, there is a continuing need for improved imaging particles for use in electrophoretic imaging systems and for methods of eliminating particles having incorrect spectral or electrical characteristics from electrophoretic imaging mixes.

SUMMARY OF THE INVENTION In addition, it has become desirable to provide a method for reclaiming the pigments from a used imaging mix which are not required in the produced image. For monochrome imaging it is necessary only to wash or brush the unused particles from the system, add a suitable amount of carrier liquid and recycle the thusly reconstituted imaging suspension into the system. The problem is more complex however when pigments of more than one color are used. If, for example, a red and a yellow pigment were used and the desired image required more yellow than red, the unused particles would be deficient in yellow and could not be recycled without balancing the pigment mix. The problem becomes far more complex when it is desired to produce images in full natural color where particles of three different colors are used. The tri-mix that is the mixture of cyan, magenta and yellow particles conventionally used in subtractive polychrome imaging will become unbalanced, that is, deficient in one or more of the colors as a natural result of the imaging process. It is desirable, therefore, to provide a system for reclaiming the particles so that the tri-mix can readily be brought into balance.

It is also desirable from an economic standpoint to reclaim the imaging suspension since the materials used have often undergone expensive process steps to make them suitable for photoelectrophoretic imaging.

It is, therefore, an object of this invention to provide an electrophoretic imaging system overcoming the abovenoted disadvantages.

It is another object of this invention to provide a method for eliminating particles having undesirable spectral or electrical characteristics from an electrophoretic imaging mix.

It is another object of this invention to provide an electrophoretic imaging system capable of producing images of improved color balance and color saturation.

It is still another object of this invention to provide a process for separating colored particles which are suitable for use in electrophoretic imaging from a particle suspension.

It is another object of this invention to provide a system for recovering pigment particles from a liquid suspension which allows relatively improved ease of reconstituting an imaging suspension.

It is another object of this invention to provide a relatively more economical photoelectrophoretic imaging system.

The above objects, and others, are accomplished by providing a system for electrophoretically separating particles in a particulate suspension. In accordance with one embodiment of this invention, the particulate suspension to be purified is placed between at least two electrodes one of which is conductive and the other of which is substantially insulating. One of the electrodes is at least partially transparent to activating electromagnetic radiation. Radiation of the wave-length to which the desired particles only should respond is projected upon the suspension through the transparent electrode. An electric field is applied across the suspension. Particles which respond to the specific wave-length of radiation used will migrate to the substantially insulating electrode. Undesired particles will be left behind on the conductive electrode. These undesired particles are those which have low sensitivity to the wave-length used or electrical characteristics which prevent their migration. This failure to migrate may be due to many characteristics such as chemical impurities which are very close in their physical and chemical properties to the desired pigment so as to make their separation by conventional chemical or physical means difiicult. Also, where the particles are composites made up of several components those particles which do not have the desired distribution of the different components will tend to remain on the conductive electrode. After exposure, the electrodes may be separated and the migrated particles removed from the insulating electrode for later use in electrophoretic imaging processes.

In a similar manner, photosensitive particles which have excessively broad photosensitive response can be eliminated. Typically, a suspension of the particles to be purified is placed between a conductive and an insulating electrode. An electric field is established between the electrodes across the suspension and light of course to which the particles should not respond is allowed to fall on the suspension through one of the electrodes. For example, cyan particles should not respond to blue light so that if blue light is allowed to fall on cyan particles in this system only those particles with excessive spectral range will migrate. After exposure, the electrodes are separated. The particles which remain on the conductive electrodes are then suitable for use in electrophoretic imaging systems.

In a preferred embodiment for reclaiming pigments from a used tri-mix the particulate suspension containing cyan, magenta and yellow particles is placed between two electrodes one of which is conductive and one of which has a substantially insulating surface. One of the electrodes is at least partially transparent to activating electromagnetic radiation. Field is applied and the suspension is exposed to radiation specific to the pigment which it is desired to remove. For example, cyan colored particles are sensitive mainly to red light; therefore, to remove these particles the suspension is subjected to light rich in the red spectra but deficient in the green and blue spectra to prevent migration of magenta and yellow particles. This may be accomplished by using a light source which is rich in the red only or a white light source in combination with a filter which will remove the blue and green wave-lengths letting the red pass. The cyan particles will then adhere to the surface of the insulating electrode. The magenta particles and the yellow particles are then removed in subsequent stages by exposure to green and blue light respectively. Where a highly volatile carrier liquid is used it may be desirable to add additional carrier liquid after each separation step.

The process of this invention may be used to separate any suitable electrically photosensitive particles. Typical electrically photosensitive particles are listed in U.S. Pat. 3,384,488 issued May 21, 1968 to V. Tulagin et al. and incorporated herein by reference.

Any suitable insulating liquid may be used as the carrier liquid for the pigment particles in this system. Typical carrier liquids include mineral oil, decane, dodecane, N-tetradecane, paraffin, beeswax or other thermoplastic materials, kerosene, long chain saturated aliphatic hydrocarbons and mixtures thereof.

The field applied is preferably between 300 and 5000 volts between the electrodes. The sign of the field depends on the particles used. It may be preferred to make the conductive electrode positive in respect to the insulating electrode for one imaging suspension and negative for another.

Where a conductive, transparent electrode is used the electrode may be made of any suitable conductive transparent material. Typical materials include conductively coated glass such as tin or indium oxide coated glass, aluminum coated glass, similar coatings on plastic substrates or conductive glass. conductively coated Mylar, a polyester, is preferred because of its strength and transparency.

The insulating surface of the insulating electrode may comprise any suitable material. Typical insulating materials include: vinyl polymers; polyolefins such as polyethylene, polypropylene, polyisobutylene; polyaromatics such as polystyrene, polyalkyds, polyvinyltoluene, polyphenylene oxides, polysulfone, polyxylylenes; polyacrylics and their esters; polyhalocarbons such as vinyl and vinylidene chlorides and fluorides; polyperfluorinated halocarbons such as polytetrafluoroethylene; polyvinyl ethers; polyvinyl acetates; polyvinylacetals and ketals such as polyvinylbutyral; phenolic resins; polyesters; polyethers; silicone resins; polycarbonates; epoxy resins; polyamides; polyimides; urethane resins; polysulfides and copolymers and mixtures thereof. Polytetrafiuoroethylene is preferred because of its excellent insulating properties and cleanability.

Although various electrode spacings may be employed, spacings of less than 1 mil and extending down even to the point where the electrodes are pressed together constitute a particularly preferred form of the invention in that they produce superior color separation results than is produced with wider spacings. This improvement is believed to take place because of the high field strength across the suspension during imaging.

BRIEF DESCRIPTION OF THE DRAWING The advantages of this improved method of reclaiming pigments will become apparent upon consideration of the detailed disclosure of the invention especially when taken in conjunction with the accompanying drawing wherein:

FIG. 1 shows a schematic representation of an electrophoretic system which may be used to separate or purify pigments and which may be also used to form images; and

FIG. 2 shows a schematic representation of a continuous electrophoretic system which may be used to separate or purify pigments and to form images.

Referring now to FIG. 1, there is seen a transparent electrode generally designated 1 which, in this exemplary instance, is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commercially available under the name NESA glass. This electrode will hereinafter be referred to as the injecting electrode. Coated on the surface of injecting electrode 1 is a thin layer 4 of finely divided photosensitive particles dispersed in an insulating liquid carrier. The term photosensitive, for the purposes of this application, refers to the properties of a particle which, once attracted to the injecting electrode, will migrate away from it under the influence of an applied electric field when it is exposed to actinic electromagnetic radiation. For a detailed theoretical explanation of the apparent mechanism of operation of electrophoretic imaging, see the above mentioned patents, the disclosures of which are incorporated herein by reference. Where this system is to be used to separate desirable from undesirable photosensitive particles, the suspension will comprise particles of a single color dispersed in a non-conductive carrier liquid. Where the system is to be used for polychromatic imaging, the suspension will comprise a mixture of two or more different colored particles in the carrier liquid. Adjacent to the electrode suspension 4 is a second electrode 5, hereinafter called the blocking electrode which is connected to one side of the potential source 6 through a switch 7. The opposite side of potential source 6 is connected to the injecting electrode 11 so that when switch 7 is closed an electric field is applied across the liquid suspension 4 between electrodes 1 and 5. A projector made up of a light source 8, an element 9 which may be a transparency or filter, and a lens 10 is provided to expose the dispersion 4 to light of a desired color or to a light image of the original to be reproduced. Where the system is used to separate desired from undesired particles, element 9 will be a suitable filter which permits only light of the desired color to reach the suspension 4. Where the system is being used to produce a polychromatic copy, element 9 will comprise a natural color transparency, such as a Kodachrome transparency. In this exemplary instance, electrode 5 is made in the form of a roller having a conductive central core 11 connected to the potential source 6. The core is covered with a layer of a blocking electrode material 12, which may be baryta paper. The particle suspension is exposed to the image to be reproduced while a potential is applied across the blocking and injecting electrodes by closing switch 7. Roller 5 is caused to roll across the top surface of injecting electrode 1 with switch 7 closed during the period of light exposure. This light exposure causes appropriate particles originally attracted to electrode 1 to migrate through the liquid and adhere to the surface of the blocking electrode, leaving behind particles on the injecting electrode surface which do not respond to the impinging light. Where the system is used for separating desirable from undesirable particles, the desirable particles may be deposited either on the injecting electrode 1 or on the blocking electrode 5, depending upon whether it is preferred that the particles respond to the impinging light or not, as discussed above. Where the system is used for image formation,

ordinarily a subtractive polychromatic image conforming to the original remains on injecting electrode 1 while unneeded particles migrate to blocking electrode 5;

Where the system is used for separating desired from undesired particles, if the light which struck the suspension is of a color which the given particles should absorb, then the particles on blocking electrode 5 are removed and saved for later use in polychromatic electrophoretic imaging. If the light which impinged upon the suspension was of a color which the particles should not absorb, then the desired particles remain on the injecting electrode 1. The particles are removed therefrom and saved for later use in electrophoetic imaging. In each case, the undesired particles are removed from the other electrode and discarded.

Where the system was used for image formation, the image may be transferred to a receiving surface and fixed by any suitable method, e.g., the methods described in copending application Ser. No. 459,860, filed May 28, 1965.

The system shown in FIG. 1 is capable of producing good separation of particles and of producing excellent images. However, since this is a batch type process and produces only small amounts of purified pigments, it may be desired to make the separation process continuous.

FIG. 2 shows a schematic representation of a continuous system for separating desired from undesired electrically photosensitive particles. Here, the injecting electrode 20 is in the form of a transparent cylinder which may be glass having a thin transparent coating of tin oxide on the exterior surface thereof. Two blocking electrode rollers 21 and 22 are arranged so as to rotate in virtual contact with injecting electrode 20. Blocking electrode rollers 21 and 22 each consists of a conductive core 23 and 24, respectively, and a surface layer of blocking electrode material, such as baryta paper, 25 and 26, respectively. Any conventional means maybe provided for rotating injecting electrode cylinder 20 and blocking electrode rollers 21 and 22 in unison. For example, synchronous motors or gearing may be provided to rotate cylinder 20 and rollers 25 and 26 at speeds such that their surfaces pass with no difference in surface speed. Projection means 27 and 28 are provided to project light of desired spectral characteristics onto the suspension at the points where blocking electrode roller 21 contacts injecting electrode cylinder 20 and at the point where blocking electrode roller 22 contacts injecting electrode cylinder 20. These projectors 27 and 28 will include suitable filters to vary the spectral characteristics of the light emitted therefrom. Variable slit means 29 and 30 may be provided to limit the amount of light falling on the suspension. The width of this slit may be varied so as to vary the quantity of light impinging on the suspension. A potential is applied between the blocking electrode rollers 21 and 22 and the injecting electrode cylinder 20 by means of potential source 31 connected to conductive core of blacking electrodes 21 and 22 and to the conductive surface of injecting electrode cylinder 20 by means of wiper 32 and switch 33. In operation, a uniform layer of the particles to be prified dispersed in an insulating carrier liquid is coated onto the surface of injecting electrode cylinder 20 from reservoir 34. To illustrate the operation of this system consider the case where reservoir 34 contains crude cyan colored particles to be refined suspended in an insulating carrier liquid. When the cyan particles reach the nip between roller 21 and cylinder 20 they are exposed to light of a color to which they should not respond. Where the particles are cyan colored, this light may be blue and green. Particles which have excessively broad spectral response will respond to the blue and green light and migrate to the surface of blocking electrode roller 21. The particles which do not have this excessive response remain on the surface of injecting electrode cylinder 20. If the insulating carrier liquid has evaporated excessively, the surface of injecting electrode cylinder 20 may be remoistened by means of spray means 35 which applied additional insulating carrier liquid. When the particle suspension reaches the nip between blocking electrode roller 22 and injecting electrode cylinder 20, the suspension is exposed to light to which the particles should respond. In the case of cyan particles the light would be red in color. Particles which have suitable photosensitivity will migrate to the surface of blocking electrode roller 22. Particles which are either insensitive or too low in sensitivity will remain on the surface of injecting electrode cylinder 20. As blocking electrode roller 22 continues to rotate, the desired particles reach doctor blade 36 which removes them from the surface of roller 22 dropping them into container 37. The particles which reach container 37 are only those which have high photosensitivity and the desired spectral response characteristics. As is further pointed out in the examples below these particles have in general superior imaging characteristics when compared to the original particles as purchased or synthesized. Meanwhile, the undesired particles which have excessive spectral response are removed from the surface of blocking electrode roller 21 by doctor blade 38 and dropped into container 39. Similarly, those particles which were relatively insensitive and remained on injecting electrode cylinder 20 reach doctor blade 40 and are removed from the injecting electrode cylinder to container 41. Patrticles reaching containers 39 and 41 may be reprocessed or discarded. Using this system, any desired quantity of crude particles may be separated into suitable and unsuitable portions. After particles of one color have been refined, particles of other colors may be similarly refined merely by changing the filters used in projection means 27 and 28 so that light of the appropriate color is produced. In each instance, light of colors to which the specific particles should not respond is projected onto the suspension at the nip between roller 21 and cylinder 20 while light of a color to which the specific particle should respond is projected onto the suspension at the nip between roller 22 and cylinder 20. Of course, each of rollers 21 and 22 and cylinder 20 could be in the form of a belt entrained around rollers, if desired.

Where the system is used to reclaim pigment particles from a used imaging mix the used suspension is placed in reservoir 34. When the suspension is brought into the nip between roller 21 and cylinder 20 where field is applied, the suspension is exposed to radiation specific to the pigment which it is desired to remove. In this exemplary instance, red light is used which causes the cyan colored particles to adhere to roller 21. The cyan particles are then removed by doctor blade 38 dropping into container 39. The suspension is then exposed to field and green light which causes the magenta particles to migrate and adhere to roller 22. Magenta particles are collected in container 37. The remaining yellow particles may be removed as shown or may be exposed to field and light at a third roller not shown.

The following examples further specifically define the present invention with respect to the method of separating suitable from unsuitable photosensitive particles and of producing images using said particles. Parts and percentages are by weight unless otherwise indicated. Examples below are intended to illustrate various preferred embodiments of the processes of the present invention.

All of the following examples are carried out in the apparatus of the general type illustrated in FIG. 1 with the particle mix coated on a NESA glass substrate through which exposure is made. The NESA glass surface is connected in series of a switch, a potential source, and the conductive center of a roller having a coating of baryta paper on its surface. The roller is approximately 2 /2 inches in diameter and is moved across the plate surface at about 4 centimeters per second. The plate employed is roughly 3 inches square and is exposed with a light intensity of about 36 foot candles as measured on the uncoated NESA glass surface. All particles which have a relatively large particle size as received commercially or as made are ground in a ball mill for about 48 hours to reduce their size in order to provide a more stable dispersion which improves the resolution of the final images. In each of the examples relating to the separation of suitable from unsuitable particles, about 7 parts of the specific particles to be refined are suspended in about parts Sohio Odorless Solvent 3440. In each case, the suspension is exposed to light of the desired color by projection utilizing appropriate color filters. In the portions of the examples in which polychromatic images are produced, about 12 parts by weight of a mixture of the three different colored particles is dispersed in about 100 parts Sohio Odorless Solvent 3440. In the imaging examples, the suspension is exposed to a natural color image by means of a conventional Kodachrome transparency. Where solvent is added enough is used to form about a 23 mil coating.

EXAMPLE I A sample of a cyan pigment, Monolite Fast Blue GS, a mixture of the alpha and beta forms of metal-free phthalocyanine, available from the Arnold Hotfman Company, is divided into two portions. The first portion is dispersed in Sohio Odorless Solvent 3440 and is coated onto the NESA glass substrate of a device similar to that shown in FIG. 1. A red filter, Wratten 29, is interposed between the light source and the NESA substrate. A potential of about 3,000 volts is maintained across the suspension during exposure as the roller electrode is passed across the suspension. Since cyan pigments should be sensitive to red light, the most photosensitive particles migrate to the roller electrode, leaving the less sensitive particles on the NESA substrate. The NESA substrate is cleaned, and the particles adhering to the roller electrode are removed and resuspended in Sohio Odorless Solvent 3440. The exposure step is repeated, however in this instance a blue filter, Wratten 47b, is in place between the light source and the NESA substrate. Since the cyan particles should not respond to blue light, after exposure only those particles having undesirably broad spectral response have migrated to the blocking electrode. The purified particles remain on the NESA substrate.

Imaging tests are then made with both the stock pigment and the purified pigment. Two dispersions are prepared by dispersing about 7 parts of each pigment in about 100 parts Sohio Odorless Solvent 3440. Each suspension is coated onto a NESA substrate and imaged as described above using a conventional black-and-white transparency between the projection lamp and the NESA substrate. In each case, an image is produced on the NESA substrate which is a positive copy of the original in cyan colored areas against the transparent background. The purified pigment exhibits higher photosensitivity and good density. The unpurified pigment produces an image having some undesirable deposition of pigment particles in background areas.

EXAMPLE H A sample of a magenta pigment, Watchung Red B, 1- (4' methyl 5' chloroazobenzene 2' sulfonic acid)- 2 hydroxy 3 naphthoic acid, OJ. No. 15865, available from E. I. du Pont de Nemours & Company, is divided into two portions. The first portion is purified as described in Example I. Here the first exposure is by means of green light through a Wratten 61 filter. Since magenta pigments should be Sensitive to green light, the more sensitive particles migrate to the blocking electrode. These particles are removed from the blocking electrode and resuspended. This suspension is then exposed to red light by means of a Wratten 29 filter. Since magenta pigments should not be sensitive to red light, the purified pigments remain on the injecting electrode with only the 9 undesirably sensitive pigments migrating to the blocking electrode.

Two suspensions are then prepared, the first consisting of about 7 parts of the stock pigment dispersed in Sohio Odorless Solvent 3440 and the second consisting of about 7 parts of the purified pigment dispersed in Sohio Odorless Solvent 3440. Each of these suspensions is imaged as described above, with a conventional black-and-white transparency placed between the light source and the NESA electrode. In each case an image conforming to the original is formed on the NESA substrate, appearing magenta-colored against the transparent background. The image produced by the purified pigments is of higher overall quality with less undesirable pigment deposition in background areas.

EXAMPLE III An example of a yellow pigment, N-2"-pyridyl-8,13- dioxodinaphtho (2,1 b; 2,3' d) furan 6 carboxamide, prepared by the process described in copending application Ser. No. 421,281, filed Dec. 28, 1964, is divided into two portions. The first portion is purified by the method generally described in Example I. A suspension prepared from this pigment is first exposed to blue light by means of a Wratten 47b filter. Since a yellow pigment should be sensitive to blue light, the preferred particles migrate to the blocking electrode leaving insensitive particles behind on the injecting electrode. The particles are removed from the blocking electrode, resuspended and re-exposed, this time to red light by means of a Wratten 29 filter. Since a yellow pigment should not be sensitive to red light, the preferred pigments remain on the NESA electrode with those pigments having excessive spectral response migrating to the blocking electrode.

Two suspensions are then prepared for imaging tests, the first consisting of about 7 parts of the stock pigment dispersed in about 100 parts Sohio Odorless Solvent 3440 and the second consisting of about 7 parts of the purified pigment suspended in about 100 parts Sohio Odorless Solvent 3440. Images are prepared from each dispersion as described above, using a conventional black-and-white transparency between the light source and the NESA electrode The image produced from the purified pigment is generally superior to that produced with the stock pigment, showing higher photosensitivity and better background.

EXAMPLE IV Two tri-mixes are prepared as follows:

(a) About 4 parts of stock Monolite Fast Blue GS, about 4 parts of stock Watchung Red B and about 4 parts of N 2' pyridyl 8,13 dioxodinaphthol (2,1- b; 2',3 d) furan 6 carboxamide as prepared in the laboratory, are dispersed in about 100 parts Sohio Odorless Solvent 3440.

(b) A second suspension is prepared as in part (a) above, except that in this instance each of the pigments has been purified as described in Examples I-III.

Each of these trimixes is coated onto a NESA substrate of a device of the sort shown in FIG. 1. Each suspension is exposed to a natural color image by means of a Kodachrome transparency placed between the light source and the NESA electrode. After exposure, each suspension is found to have produced a full color image' on the NESA electrode conforming to the original. The purified pigments are found to be more sensitive than unpurified pigments. The image produced by the purified pigments is found to have superior color balance and little deposition of unwanted pigments in background areas.

EXAMPLE V A succession of images are prepared as in I'II(b). After formation of each image the unused particles adhering to the blocking electrode are removed and resus- 10 pended in Sohio Odorless Solvent 3440. The pigments are separated and reclaimed as follows.

The suspension is coated onto the NESA substrate and exposed to field and red light as in Example I. The cyan pigment adheres to the roller electrode. The cyan pigment is then removed from the roller. Additional Sohio is sprayed onto the injecting electrode having the remaining two pigments on it. The cleaned roller is then passed across the imaging suspension while the suspension is exposed to field and green light as in Example II. The magenta particles adhering to the roller electrode are removed. Additional Sohio is sprayed onto the injecting electrode. The cleaned roller is again passed across the imaging suspension while the imaging suspension is exposed to field and blue light as in Example III. The yellow particles adhere to the roller electrode and are removed. About 4 parts of each of the thusly reclaimed pigments are then suspended in parts Sohio. The reconstituted imaging suspension is exposed to a full color Kodachrome transparency in Example IV. The image is found to have excellent color balance and little deposition of unwanted pigments in background areas.

Although specific components and proportions have been described in the above examples, other suitable materials, as listed above, may be used with similar results. In addition, other materials may be added to the particles, or to the particle suspensions to synergize, enhance, or otherwise modify their properties. Typically, the particles or suspension may be dye sensitized or electrially sensitized, if desired.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. A method of separating electrically photosensitive particles from a polychromatic photoelectrophoretic imaging suspension which comprises:

(a) forming a layer of a suspension of electrically photosensitive particles between a first transparent conductive electrode and a second electrode having an insulating surface, said particles comprising particles of a first color and particles of a second color, said particles of said first color having a spectral response curve which does not substantially overlap the spectral response curve of the particles of a second color;

(b) subjecting said suspension to an electric field applied between said first and said second electrodes;

(c) exposing said suspension to electromagnetic radiation of a first spectra to which only said particles of said first color are sensitive until at least a portion of said particles of said first color adhere to said second electrode; and,

(d) separating said first and said second electrodes.

2. The method of claim 1 and further including the steps of:

(e) contacting the free surface of said suspension remaining on said first electrode with a third electrode, said third electrode having an insulating surface;

(f) subjecting said suspension to an electric field applied between said first and said third electrodes;

(g) exposing said suspension to electromagnetic radiation of a second spectra to which only said particles of said second color are sensitive until at least a portion of said particles of said second color adhere to said third electrode; and

(h) separating said electrodes.

3. A method of separating electrically photosensitive particles from a polychromic photoelectrophoretic imaging suspension which comprises:

(a) forming a layer of a suspension of electrically photosensitive particles between a first transparent conductive electrode and a second electrode having an insulating surface, said particles comprising particles 1 l of a first color, particles of a second color and particles of a third color, each particle of one color having a spectral response curve which does not substantially overlap the spectral response curves for the remaining particles;

(b) subjecting said suspension to an electric field applied between said first and second electrodes;

(c) exposing said suspension to electromagnetic radiation of a first spectra to which said particles of said first color are sensitive until at least a portion of said particles of said first color adhere to said second electrode;

((1) separating said first and said second electrodes;

(e) contacting the free surface of said suspension remaining on said first electrode with a third electrode, said third electrode having an insulating surface;

(f) subjecting said suspension to an electric field applied between said first and said third electrodes;

(g) exposing said suspension to electromagnetic radiation of a second spectra to which only said particles of said second color are sensitive until at least a portion of said particles of said second color adhere to said third electrode; and

(h) separating said electrodes.

4. The method of claim 3 including the steps of (i) contacting the free surface of said suspension remaining on said first electrode with a fourth electrode, said fourth electrode having an insulating surface;

(j) subjecting said suspension to an electrical field applied between said first and said fourth electrodes;

(k) exposing said suspension to electromagnetic radiation of a third spectra to which said particles of said third color are sensitive until at least a portion of said particles of said third color adhere to said fourth electrode; and

(l) separating said electrodes.

5. The method of claim 3 wherein said suspension comprises cyan particles responsive mainly to red light, magenta particles responsive mainly to green light and yellow particles responsive mainly to blue light dispersed in an insulating carrier liquid.

6 The method of claim 3 wherein prior to step (e) additional liquid is added to the suspension.

7. The method of claim 4 wherein prior to step (i) additional liquid is added to the suspension.

References Cited UNITED STATES PATENTS 3,446,722 5/1969 Stein et al 204-180 JOHN H. MACK, Primary Examiner A. C. PRESCOTT, Assistant Examiner U.S. Cl. X.R. 204-181, 300 

